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Date:   Tue, 12 Apr 2022 17:40:58 +0800
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Ard Biesheuvel <ardb@kernel.org>, Will Deacon <will@kernel.org>,
        Marc Zyngier <maz@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of
 ARCH_KMALLOC_MINALIGN
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On Tue, Apr 12, 2022 at 10:32:58AM +0100, Catalin Marinas wrote:
>
> At a quick grep, most cra_alignmask values are currently 15 or smaller.
> I'm not convinced the driver needs to know about the CPU cache
> alignment. We could set cra_alignmask to CRYPTO_MINALIGN but that would
> incur unnecessary overhead via function like setkey_unaligned() when the
> arch_kmalloc_minalign() was already sufficient for DMA safety.
> 
> Maybe I miss some use-cases or I focus too much on DMA safety.

The alignment was never designed for DMA.  It's mainly for CPUs
that provide accelerated instructions that require a certain
amount of alignment, most commonly 16 bytes.

Therefore CRYPTO_MINALIGN was never meant to be a guarantee for
DMA alignment.  Any drivers relying on this is simply broken.

I understand that on ARM for historical reasons you have had a
situation that CRYPTO_MINALIGN happened to be large enough to
guarantee DMA alignment.  I totally agree with your patch to
try to fix this but it should not penalise other architectures
and raise the CRYPTO_MINALIGN unnecessarily.

I think if CRYPTO_MINALIGN makes those drivers work then so
should cra_alignmask.  And that would be a relatively easy
patch to do.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt